The invention relates to liquid filters, in particular oil filters or fuel filters for an internal combustion engine, comprising a housing having a removable cover, an inlet for liquid to be filtered which opens into an unfiltered side of the liquid filter, an outlet for filtered liquid which starts from a filtered side of the liquid filter, a replaceable filter insert, which separates the unfiltered side and the filtered side from one another and which comprises a hollow cylindrical filter material element enclosed at the end faces by two end discs, and a filter bypass valve, consisting of a valve seat arranged on the filter insert and a valve body which interacts with the valve seat and is acted upon by a preload force in the closing direction, wherein two closed sealing contours arranged one inside the other as viewed in the radial direction are provided between the valve seat and the valve body, and wherein at least one bypass channel, which connects the unfiltered side and the filtered side of the liquid filter while bypassing the filter material element, passes through the valve seat between the sealing contours. The invention further relates to a filter insert for such liquid filters.
A liquid filter of the aforementioned type is known from DE 200 04 431 U1. The known liquid filter has a housing structure in which the filter insert is installed. The housing structure may constitute a separate housing or may be an integral component for example of an internal combustion engine. The filter insert is of cylindrical construction, wherein the filter bypass valve is accommodated in the region of one of the end discs of the filter insert. To this end the valve seat is accommodated in the end disc and the valve body is axial movably mounted in the end disc of the filter insert. On the other hand, in the housing structure a supporting dome is provided which serves for mounting of a spring which is responsible for the initial tension of the filter bypass valve, thereby defining the opening pressure. The valve body is fastened movably in the filter insert in such a way that it is also replaced when the filter element is changed. In this way an unacceptably high contamination of the valve body, for example by soot deposits, can be avoided. The spring remains in the supporting dome when the filter element is replaced.
In the known liquid filter it is regarded as disadvantageous that due to the integration of the valve body the filter insert is relatively complex and therefore expensive. It has also proved unfavorable that the guiding of the valve body in the filter insert is difficult and that twisting and leaks of the bypass valve caused thereby cannot be reliably ruled out. This problem is also caused by the fact that the spring preloading the valve body in the closing direction of the filter bypass valve is part of the filter housing and that thus at every change of the filter insert it must be ensured that the spring and the valve body are brought into exact abutment on one another.
The document DE 299 15 843 U1 shows a filter, such as an oil filter, with a filter housing with a cover, a filter insert and a central standpipe with drain valve function. In addition, according to claim 7 a filter bypass valve is provided, the valve body of which is axially movably guided on the cover and is preloaded in the closing direction by a spring supported on the cover. The valve seat is disposed in the upper end disc of the filter insert. The force for opening the valve is generated here by the effect of the oil pressure on a flange of the valve body guided in a sealed manner in the cover.
In this known liquid filter it is regarded as disadvantageous that the valve body in its axial movement is subject to relatively substantial friction because it is sealed against the cover by means of a radial seal. This friction can lead to malfunctions of the filter bypass valve or, in the event that the seal of the valve body sticks on the guide surface in the cap, can even lead to failure of the filter bypass valve function.
Therefore the object of the present invention is to provide liquid filters of the aforementioned type which avoid the disadvantages of the prior art and in which a simpler, more cost-effective filter insert can be used and reliable functioning of the filter bypass valve is ensured. Moreover a corresponding filter insert should be provided.